<!DOCTYPE html>
<html lang="en">
  <head>
    <meta charset="UTF-8" />
    <meta http-equiv="X-UA-Compatible" content="IE=edge" />
    <meta name="viewport" content="width=device-width, initial-scale=1.0" />
    <title>Document</title>
   
  </head>
  <body>
    <div id="app">
        firstName:
         <input type="text" v-model="firstName">
         lastName:
         <input type="text" v-model="lastName">

         =>

         {{fullName}}
    </div>

    <script src="./vue.global.js"></script>
    <script>
      const { createApp } = Vue;
      const app = createApp({
        data() {
          return {
            firstName: '',
            lastName: '',
            fullName: ''
          };
        },
       
        watch: {
          firstName(val){
            this.fullName = val + '.' + this.lastName
          },
          lastName (val) {
            this.fullName = this.firstName + '.' + val
          }
        }
      });
      app.mount("#app");
    </script>
  </body>
</html>
